Plasma deposition refers to any of a wide variety of processes in which a plasma is used to assist in the deposition of thin films or coatings onto the surfaces of objects. For example, plasma deposition processes are widely used in the electronics industry to fabricate integrated circuits and other electronic devices, as well as to fabricate the magnetic tapes and disks used in audio, video, and computer applications. Plasma deposition processes may also be used to apply various coatings to glass surfaces in the optics and architectural glass industries. Such plasma deposition processes may also be used in reverse, i.e., to remove material from the surfaces of objects, in which case they are usually referred to as plasma etching or plasma cleaning processes.
Regardless of whether the plasma process is used for deposition or cleaning, the plasma is usually created by subjecting a low-pressure process gas (e.g., argon) contained within a vacuum chamber to an electric field created between two electrodes. The electric field ionizes the process gas, creating the plasma. In the case of a sputter deposition plasma process, the ionized process gas atoms comprising the plasma impact the surface of the material (i.e., target) that is to be deposited on the object (i.e., substrate). As a result of the ion impacts, atoms of the target material are dislodged or sputtered from the surface of the target material and are released into the vacuum chamber. The substrate is usually positioned with respect to the target so that a majority of the sputtered atoms from the target material deposit themselves onto the surface of the substrate.
While sputter deposition processes of the type described above may be used to deposit metals or metal alloys (e.g., aluminum, nickel or cobalt) onto the substrate, they may be used to deposit compound compositions as well. Reactive sputter deposition is the name usually given to reactive sputtering processes which involve the sputtering of the target material in a reactive gas mixture in order to deposit a compound thin film comprising the sputtered target material and the reactive species. A wide variety of compounds, such as SiO.sub.2, Al.sub.2 O.sub.3, Si.sub.3 N.sub.4, and TiO, can be deposited by reactive sputter deposition.
While reactive sputtering processes are known and have been used for years, they continue to be plagued by several problems. For example, it is common for the anode to slowly accumulate a layer of the compound material that is to be deposited on the surface of the substrate. Since the deposited compound material is usually an electrical insulator (e.g., an oxide), the gradual accumulation of the compound material on the anode will eventually result in a decrease in anode efficiency, with a commensurate decrease in the overall efficiency of the process. Worse yet, if the process chamber itself is connected as the anode, the entire chamber tends to accumulate a layer of the compound material, thereby necessitating the need for frequent cleaning and/or replacement of the entire process chamber.
The accumulation of the compound material on the anode can also cause other problems. For example, since the electrons seeking the anode take the path of least resistance, they tend to "find" certain low resistance areas on the anode where the accumulated coating is relatively thin. Since such low resistance areas are usually fairly small, the incoming electrons tend to heat the low resistance area to high temperatures. Such areas are referred to herein as "hot spots." In extreme cases, the temperature of a hot spot may increase to the point where it becomes incandescent. While the formation of hot spots is generally undesirable in that they can melt or partially melt the anode, they can be particularly serious where the anode is the process chamber, i.e., the hot spot may cause a "burn through" or breach in the process chamber wall.
Still another problem associated with reactive sputtering systems is that it is often difficult to establish (i.e., "light") and/or maintain the plasma due to the presence in the process chamber of the reactant gas species. One method of overcoming this problem has been to supply additional electrons to the process chamber by use of a separate electron emitter, such as a hot filament or thermionic emitter. Unfortunately, however, hot filaments are prone to rapid deterioration in the highly reactive atmosphere within the process chamber.
Consequently, a need exists for a reactive plasma processing system that is resistant to the formation on the anode of the compound material that is to be deposited on the surface of the substrate. Still other advantages could realized if such a system could provide increased ionization levels, thus making it easier to light and maintain the plasma.